Silicone rubbers are excellent in electric insulation, heat resistance, weatherability, and flame retardancy, and they have been used as a coating material on a fixing roll including heat roll and pressure roll in a copier machine or laser beam printer. With the increase in the speed of the copying and popularization of color copying, the fixing roll is required to have lower hardness. Nowadays, rolls prepared from a highly heat conductive silicone rubber coated with a fluororesin are often used in replacement of the conventional metal or fluororesin rolls which failed to cope with the high speed and the color copying. More specifically, the rubber used for the heat roll is required to have a high thermal conductivity to reduce the warm-up time and also, to save the energy required for the machine operation. This roll should also have low compression set since the roll is constantly exposed to a high temperature of 150 to 230° C.
In the meanwhile, the thickness of the rubber layer has also been reduced to thereby reduce the warm-up time required for the temperature increase as well as the printer size. Under such situation, in addition to the conventional rolls prepared by forming a rubber layer on the mandrel, fixing belts comprising an endless belt made of a thin sheet of a metal or a heat resistant resin having formed thereon a rubber layer and a release layer has also be used.
However, the silicone rubber does not have a high thermal conductivity, and a filler having a high thermal conductivity has been added to the silicone rubber to fulfill such requirement. Exemplary silicone rubbers include those disclosed in JP-A S58-219259, JP-A H03-221982, and JP-A H10-39666. The silicone rubbers are those prepared by adding a thermally conductive filler such as silica, alumina, or magnesium oxide to the conventional silicone rubber. These fillers, however, have been associated with the problem of the silicone rubber deterioration when the filler content is increased or the product is used at a high temperature. As an alternative filler, JP-A 2003-208052 discloses the use of silicon carbide for the thermal conductivity filler. However, the use of silicon carbide had the problem that the compression set of the resulting product varied so much depending on the particle size and the treating method.
Accordingly, there is a demand for the development of a silicone rubber composition having a highly thermal conductivity which can be formed into a silicone rubber exhibiting high thermal conductivity simultaneously with the low compression set, and which also experiences reduced change of physical properties under high temperature conditions.